Steel with High Wear Resistance, Hardness and Corrosion Resistance as well as Low Thermal Conductivity

ABSTRACT

This invention relates to a steel having high wear resistance, a high degree of hardness, good corrosion resistance and/or low thermal conductivity. A hardness of a steel is at least 56 HRC in a hardened state. In order to obtain this hardness, in a microstructure of the steel in total at least 30% wt. of hard phases are present which in addition to TiC particles include further carbide particles, oxide particles or nitride particles. Content of the TiC particles in the steel is at least 20% wt. The hard-phase particles are embedded in a matrix which includes (in % wt.) 9.0-15.0% Cr, 5.0-9.0% Mo, 3.0-7.0% Ni, 6.0-11.0% Co, 0.3-1.5% Cu, 0.1-2.0% Ti, 0.1-2.0% Al, the remainder being iron and unavoidable impurities.

The invention relates to a steel for applications which require high wear resistance, a high degree of hardness, good corrosion resistance and/or low thermal conductivity.

When content details of steel alloys are stated below, these are based on the weight in each case, unless expressly stated otherwise.

Steels with the above mentioned profile of properties are particularly suitable for producing cutting tools, die plates, sieves, moulds and comparable components for machines which are required in the plastics processing industry.

A typical area of use here is machines for reproducing or recycling plastic products which are melted down to a melt, in order to return them to the processing cycle. In order to form a pellet from the melt, the melt is extruded through a die plate which it exits in a plurality of single strands. The single strands solidify and are then reduced in size to individual pellet grains by means of suitable knives rotating close to the die plate.

In order to accelerate the solidification process, extruding the plastic melt through the die plate and reducing it to small pieces can be carried out underwater. This process is known as “underwater pelletizing” in the plastics industry.

Both the knives used for reducing the plastics to small pieces and the die plates used for forming the single strands to be reduced to small pieces by the knives must have good corrosion resistance, due to the corrosive environment to which they are exposed in use, and are at the same time exposed to a high level of abrasive wear. Particularly for the “die plate” application the thermal conductivity of the steel, from which the die plate has been produced in each case, should at the same time be low, so that not too much heat is withdrawn from the plastic melt coming into contact with the respective die plate and a premature solidification of the melt occurs, which would result in the holes of the plate becoming blocked. There is this requirement particularly when the die plate is a so-called “micro die plate” with hole diameters of less than 1 mm.

A known steel provided for these purposes is known under the material number 1.2379 (AISI designation: D2). It contains, in addition to iron and unavoidable impurities, (in % wt.) 1.55% C, 12.00% Cr, 0.80% Mo and 0.90% V.

Another steel which is also widely used in plastics recycling is standardised under the material number 1.3343 (AISI designation: M2). It contains, in addition to iron and unavoidable impurities, (in % wt.) 0.85-0.9% C, 0.25% Mn, 4.1% Cr, 5.0% Mo, 1.9% V and 6.4% W.

The martensitic steel standardised under the material number 1.4110 (AISI designation: 440A), which, in addition to iron and unavoidable impurities, contains (in % wt.) 0.6-0.75% C, max. 1% Mn, max. 1% Si, max. 0.04% P, max. 0.03% S, 16-18% Cr, and max. 0.75% Mo should stand up to the highest wear demands. After a suitable heat treatment this steel attains a hardness of at least 60 HRC.

A steel known under the trade name “Ferro-Titanit Nikro 128” which was specifically created for producing components which are used when processing abrasive plastics, contains, in addition to iron and unavoidable impurities, (in % wt.) 13.5% Cr, 9% Co, 4% Ni and 5% Mo. The proportion of titanium carbide in the microstructure of the steel composed in this way is 30% wt., which corresponds to a percentage by volume of approximately 40% vol. TiC.

The known steel produced by powder metallurgy, after it has been annealed over two to four hours in a vacuum at 850° C. and subsequently quenched, in which it is exposed to a nitrogen atmosphere at a pressure of 1-4.5 bar, attains an annealed hardness of approximately 53 HRC, which can be increased to a maximum hardness of approximately 62 HRC by means of a subsequent precipitation hardening treatment, in which the steel is aged over six to eight hours at 480° C. Die plates, pelletizing knives, injection moulding nozzles and screws, rings and other pressing tools for processing abrasively-acting plastics, as well as components for pumps, filling heads and ring knives which are required for can filling machines, are typically produced from this steel (see data sheet “Ferro-Titanit Nikro 128”, contained in the brochure “Ferro-Titanit-Die Härte aus Krefeld” [Ferro-Titanit-The hardness from Krefeld], 06/2001, published by Deutsche Edelstahlwerke GmbH).

Finally, a steel has been proposed by Horst Hill in his dissertation “Neuartige Metallmatrixverbundwerkstoffe (MMC) zur Standzeiterhöhung verschleißaeanspruchter Werkzeuge in der polymerverarbeitenden Industrie” [New types of metal matrix composites (MMCs) for increasing the service life of tools subject to wear in the polymer processing industry” ], Bochum Univ. Diss. 2011, published by the Chair for Materials Engineering, Ruhr-Universität Bochum, ISBN 978-3-943063-08-0, which consists (in % wt.) of 13.5% Cr, 1.0% Mo, 9.0% Ni, 5.5% Co, 1.0% Cu, 2.0% Ti and 1.25% Al, with the remainder iron and unavoidable impurities. The TiC proportion in the microstructure of this steel also amounts to 30% wt. However, in addition, 5% wt. NbC is present in the microstructure as a hard phase.

The steel composed in such a way when it was being produced on a laboratory scale gave rise to hopes of a promising potential. However, its production on an operationally reliable industrial scale has turned out to be problematical.

Against this background, the object of the invention was to create a steel which can be produced on an industrial scale applying conventional methods and which has an optimised profile in terms of its properties. Practice-oriented uses of such a steel should also be stated.

In relation to the steel, this object is achieved by such a steel according to the invention having the features specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and like the general concept of the invention are explained in detail below.

By means of the invention, a steel is available for applications which require high wear resistance, a high degree of hardness, good corrosion resistance and/or low thermal conductivity.

The steel according to the invention obtains a hardness of at least 56 HRC in the hardened state and contains in its microstructure in total at least 30% wt. of hard phases which in addition to the TiC particles consist of carbide particles, oxide particles or nitride particles. At the same time, the content of TiC particles in the steel according to the invention is at least 20% wt.

According to the invention, the hard phases are embedded in a matrix which consists (in % wt.) of

-   -   9.0-15.0% Cr,     -   5.0-9.0% Mo     -   3.0-7.0% Ni,     -   6.0-11.0% Co,     -   0.3-1.5% Cu,     -   0.1-2.0% Ti,     -   0.1-2.0% Al,     -   with the remainder iron and unavoidable impurities.

The components of a steel according to the invention are set such that it meets the highest requirements put on steels which are used in the plastics processing industry.

Correspondingly, a steel according to the invention is particularly suitable for the production of components for reproducing and for recycling plastic products. Thus, for example, die plates, in particular micro-pelletizing die plates, required for pelletizing melts formed from abrasive plastics can be produced from steel according to the invention, which themselves have optimum use properties even if their hole openings are micro-finely formed, in order to produce correspondingly fine-grained pellets. Knives for reducing plastic parts to small pieces can likewise be produced from steel according to the invention. Such knives are, as already explained above, also required when producing pellets from melted plastic strands as are produced by means of die plates of the type explained above in pelletizing installations.

In order to provide the profile of properties required for this purpose, a steel according to the invention contains at least 20% wt. TiC which is embedded in a matrix which through precipitation formation contributes to the hardenability of the steel and which, at the same time, is chosen such that low thermal conductivity of less than 35 W/mK is guaranteed irrespective of the respective heat treatment state.

The passive current density of the steel according to the invention is less than 5 μA/cm², measured in oxygen-free 0.5 molar sulphuric acid with a potential change speed of 600 mV/h against a calomel reference electrode at 20° C. Therefore, steel according to the invention with a high degree of hardness and optimised wear resistance has a resistance to corrosion which is comparable to the resistance to corrosion of conventional austenitic stainless steels.

The E-modulus of steels according to the invention determined by means of ultrasonic measurement subject to the sound-propagation velocity is at a temperature of 20° C. more than 270 GPa, in particular more than 300 GPa, so that the steel according to the invention or the components produced from it also reliably fulfil the highest requirements with regard to their strength.

The thermal coefficient of expansion of steel according to the invention determined by means of a dilatometer lies in the temperature range important for applications for which steels according to the invention are provided of 20° C. to 600° C. at 7×10⁻⁶/K to 12×10⁻⁶/K.

Due to the presence of a sufficient amount of the extremely hard, thermodynamically stable TiC particles, which have a low density with low thermal conductivity, in combination with the steel matrix provided according to the invention which also attains a high degree of hardness, maximised wear resistance is obtained with, at the same time, low thermal conductivity. Optimally, to that end, the steel according to the invention contains at least 20% wt. corresponding to approximately 30% vol. TiC, or at least 28% wt. TiC, in particular at least 30% wt. TiC. However, the TiC content should not exceed an upper limit of 45% wt. In this way, it can be ensured that steel according to the invention can be operationally reliably produced and processed further. Although excessive hard phase contents result in increased hardness and wear resistance, the thermal expansion is reduced, which considerably hampers composite production with steel substrates. In addition, a higher hard phase content means that the material becomes more brittle and more susceptible to cracks. At the same time, the machining possibilities are significantly reduced with excessive hard phase contents. An advantage of steel according to the invention here is that it can also be conventionally machined.

The fact that according to the invention in addition to the TiC particles further hard phases are present in the steel matrix, so that the percentage by volume of the hard phases in the microstructure of the steel is in total at least 30% wt, also contributes to optimising the hardness and wear resistance of a steel according to the invention. This can be effected by separately adding carbide, nitride or oxide particles during production of the steel. Alternatively or additionally to this, the elements (Ni, Al, Ti) forming the proportions by weight of the precipitations can be set within the input requirements according to the invention such that in the course of the production steps carried out during production of the steel a sufficient amount of precipitations increasing hardness is reliably formed in the matrix.

Compared to the steel known from the dissertation of H. Hill already mentioned above, in the case of the steel according to the invention the contents of Mo and Co have been considerably increased and the contents of Ni and Ti have been considerably reduced. In addition, the input requirements for the Cu, Al, TiC and NbC contents of an alloy according to the invention have been varied compared to the known steel. By setting the alloy contents according to the invention, a steel was able to be successfully produced on an industrial scale which has a high hard-phase proportion which is embedded in a matrix also having a high degree of hardness. Starting from the known steel concepts, this required extensive investigations and tests because the mode of action and interactions of the individual elements and phases in the case of steels of the type in question here are very complex. The steel according to the invention obtained in this way with its high wear resistance, high degree of hardness, good corrosion resistance and low thermal conductivity has an optimised combination of properties.

The precipitations which are formed in the steel matrix of the steel according to the invention are intermetallic precipitations which, above all, the elements Ni, Al and Ti are involved in forming. These elements form Ni₃Al and Ni₃Ti or mixed forms. These intermetallic phases are present in the microstructure with grain sizes of the order of 10 nm and are not included in the total hard-phase content. Due to their small size they do not make a great contribution to resistance against abrasive wear compared to the coarse hard-phase particles, as are embedded according to the invention in the matrix of the steel according to the invention, but the intermetallic precipitations bring about an increase in the hardness and strength of the metal matrix and, in this way, also contribute to improvement in the performance characteristics.

Chromium is present in contents of 9.0-15.0% wt. in the steel according to the invention, in order to guarantee the required corrosion resistance. Optimally, the Cr content amounts to 12.5-14.5% wt. for this purpose.

Molybdenum is contained in contents of 5.0-9.0% wt. in the steel according to the invention, in order, on the one hand, to guarantee sufficient resistance to corrosion, in particular with regard to the hole corrosion, and, on the other hand, to support the formation of intermetallic phases, by means of which the hardness of the steel matrix, in which the hard phases are embedded, is increased. Optimally, the Mo content of the steel according to the invention is 6.5-7.5% wt.

Cobalt is contained in contents of 6.0-11.0% wt. in the steel according to the invention, in order, on the one hand, to increase the martensite start temperature and, on the other hand, to reduce the solubility of Mo in the metal matrix. In this way, the Mo contained in the steel matrix according to the invention can participate more strongly in the formation of intermetallic phases. Optimally, the Co content of the steel according to the invention is 8.0-10.0% wt.

Copper is contained in contents of 0.3-1.5% wt. in the steel according to the invention, in order to accelerate the precipitation hardening. Optimally, the Cu content of the steel according to the invention is 0.5-1.0% wt.

Nickel is contained in contents of 3.0-7.0% wt. in the steel according to the invention. Nickel is required in a sufficient amount in the steel matrix in order to stabilise the austenitic phase during a solution annealing operation which is typically carried out at approximately 850° C. This is particularly important if the material according to the invention is quenched starting from the solution annealing temperature. As a result of the presence of nickel, the austenite is stabilised here to the extent that martensite is reliably formed during quenching. If too little nickel is present in the steel matrix provided according to the invention, then this effect is no longer achieved with the necessary reliability. If, on the other hand, too much nickel is present in the steel matrix, no martensite forms, since the austenitic phase is then also stable at room temperature. The second function of nickel in the steel according to the invention is precipitation hardening by forming intermetallic phases with elements like Al and Ti. Therefore, the contents of Ni, Al and Ti are geared to one another in the steel matrix of the steel according to the invention in such a way that, on the one hand, there is formation of martensite and, on the other hand, the precipitation hardening is made possible. Optimally, the Ni content of the steel according to the invention is 4.5-5.5% wt. for this purpose.

Titanium is present in contents of 0.1-2.0% wt. in the steel according to the invention, so that, as already mentioned above, in combination with Ni the precipitation hardening is made possible. Optimally, the Ti content of the steel according to the invention is 0.8-1.2% wt. for this purpose.

Aluminium is also present in contents of 0.1-2.0% wt. in the steel according to the invention, so that in combination with Ni the precipitation hardening is brought about. Optimally, the Al content of the steel according to the invention is 1.0-1.4% wt. for this purpose.

The steel according to the invention can be hardened with extremely low warping, since titanium carbide has a low thermal expansion and no transformation.

The wear resistance of the steel according to the invention is increased by adding up to 4.5% wt. of NbC particles. At the same time, the NbC particles have a lower thermal conductivity than TiC, which has a favourable effect on the performance characteristics of the steel according to the invention. In addition, TiC and NbC are isomorphic carbides and are therefore miscible with one another. In the case of diffusion reactions this results in the formation of composite carbides. As a result of this, compared to only using TiC, there is a change in the valence electron concentration and hence formation of vacancies in the interstitial lattice of the carbon. In this way too, the thermal conductivity of the steel according to the invention is lowered and the fitness for purpose improved. This effect can in particular be achieved if at least 2.0% wt. of NbC is present in the steel according to the invention. There is an optimum effect if the NbC content is 2.0-3.0% wt.

By producing the steel according to the invention in a conventional way by powder metallurgy, it can be ensured that its microstructure is free of segregations and fibre orientations. The carbide, nitride and oxide particles used as hard phases according to the invention are already provided as “complete” particles during powder-metallurgical production.

Both the sintering and the HIP (Hot Isostatic Pressing) routes can be used for powder-metallurgical production. By way of example, supersolidus liquid-phase sintering based on gas atomised steel powder is also suitable for producing steels according to the invention.

A description of the production steps usually applied during the powder-metallurgical production of steels of the type in question here can be found, for example, in Foller, M.; Meyer, H.; Lammer, A.: Wear and Corrosion of Ferro-Titanit and Competing Materials. In: Tool steels in the next century: Proceedings of the 5^(th) International Conference on Tooling, Sep. 29-Oct. 1, University of Leoben, Austria, 1999, pages 1-12, in H. Hill, S. Weber, W. Theisen, A. van Bennekom, Optimierung korrosionsbeständiger MMC mit hohem Verschleißwiderstand [Optimising corrosion-resistant MMCs with high wear-resistance], 30^(th) Hagen Symposium, 24-25, Nov. 2011 or in the dissertation of Horst Hill already mentioned above.

The steel according to the invention can be subjected to a conventional heat treatment for setting its mechanical properties, in which it is heated for 2-4 hours, subsequently quenched in a nitrogen atmosphere subjected to a pressure of 1-4.5 bar and finally aged over 6-8 hours at 480° C. After such a heat treatment, steel according to the invention consistently has a hardness of more than 62 HRC. By heating taking place in a vacuum and quenching being carried out in an inert gas atmosphere, negative influence zones in the edge region of the semi-finished product in each case formed from the steel for the heat treatment can be prevented. If the heat treatment is limited to a soft annealing operation at 850° C. over 2-4 hours, then the steel according to the invention has a hardness of more than 50 HRC.

The invention is explained in more detail below by means of exemplary embodiments. The figures show:

FIG. 1 a part of a scanning electron micrograph of a section of a sample according to the invention;

FIG. 2 a diagram, in which the results of the measurement of the thermal conductivity of steel samples produced according to the invention and produced for comparison are illustrated;

FIG. 3 a diagram with the result of a current density potential measurement carried out on steel samples produced according to the invention and produced for comparison;

FIG. 4 a diagram which reproduces the result of a dilatometer measurement on a sample produced from steel according to the invention.

The steel E according to the invention and the known steel V have been produced for comparing the properties of a steel according to the invention, which is intended for producing die plates or knives for an underwater pelletizing machine, to the properties of a known steel intended for the same purpose. The composition of both steels E and V is indicated in Table 1.

The composition of steel V corresponded to the composition of the steel known under the designation “Ferro-Titanit Nikro 128” documented for example in the publication already mentioned above. The production steps completed during powder-metallurgical production of both steels E, V corresponded to the production steps which are normally carried out during powder-metallurgical production of the steel “Ferro-Titanit Nikro 128”. They are explained in the technical literature already mentioned above.

After powder-metallurgical production is complete, samples PE1, PV1 of the steels E and V were subjected to a heat treatment which likewise corresponded to the heat treatment carried out in a standard manner for the steel Ferro-Titanit Nikro 128. To that end, the probes PE1 and PV1 were firstly held at a temperature of 850° C. in a vacuum over a period of two to four hours and were then quenched in a nitrogen atmosphere subjected to a pressure of 1-4.5 bar.

Subsequently, a precipitation hardening treatment was carried out, in which the samples PE1, PV1 were each aged for six to eight hours at a temperature of 480° C.

FIG. 1 shows a part of a scanning electron micrograph of a section of a sample PE1 of the steel E according to the invention heat-treated in such a way in a standard manner. The metal matrix can be identified by the light areas, whereas the TiC inclusions surrounded by the matrix are reproduced dark in colour.

Other samples PE2, PV2 consisting of the steels E and V were subjected to a soft-annealing operation at 850° C. also extending over 2-4 hours.

The hard-phase contents were determined in the samples PE1, PV1, PE2, PV2. In the case of the samples PE1, PE2 produced from the steel according to the invention they on average amounted to more than 30% wt., whereas the samples PV1, PV2 produced from the comparison steel V had on average only 30% wt. of hard phases.

Five hardness measurements were carried out according to DIN EN ISO 6508-1 to determine the hardness of the different samples PE1, PE2, PV1, PV2. The average values of the measured values collected in this way for the probes PE1, PE2, PV1, PV2 are indicated in Table 2. It becomes apparent that the hardness of the samples PE1, PE2 according to the invention was in each case higher than the hardness of the comparison samples.

In addition, the temperature-dependent thermal conductivity λ(T) was determined by means of the indirect method at room temperature, 100° C., 200° C. and 300° C.:

λ(T)=a(T)×ρ(T)×cρ(T)

-   with a(T): thermal diffusivity, measured by means of Laserflash, as     explained in Linseis Messgeräte GmbH: Instruction Manual LFA     1250/1600-Laser Flash: Thermal constant analyser, 2010, or ASTM     International E 1461-01: Standard Test Method for Thermal     Diffusivity by the Flash Method, 2001; -   ρ(T): the density of the respective sample, measured with the     dilatometer; -   cρ(T): the specific isobaric heat capacity of the sample, determined     by differential scanning calorimetry (“DSC”).

The result of this investigation for the two samples PE1 and PV1 is illustrated in FIG. 2. It becomes apparent that the thermal conductivity in the case of the sample PE1 produced from the steel E according to the invention was in each case lower than with the sample PV1 which was produced from the comparison steel V. The low thermal conductivity of the sample PE1 according to the invention is advantageous with regard to the intended purpose of the steels E and V here.

The TiC content of the samples PE1, PE2 was, as indicated in Table 1, in each case more than 30% wt.

The density of the samples PE1, PE2 produced from the steel E according to the invention was 6.55 g/cm³, whereby the theoretical density was reached. As becomes evident from FIG. 1, the microstructure has no residual porosity.

The result of a current density potential measurement carried out on samples PE1 produced from the steel E according to the invention and on samples PV1 produced from the comparison steel V is illustrated in FIG. 3. In this figure, the current density potential curve determined for the samples PE1 is illustrated as a continuous line and the current density potential curve determined for the samples PV1 is illustrated as a broken line. The current density potential curves were measured in oxygen-fee 0.5 molar sulphuric acid with a potential change speed of 600 mV/h against a calomel reference electrode at 20° C. In addition, the passive current densities determined for the samples PE1 according to the invention were in each case below 5 μA/cm².

The elasticity modulus was determined as 318 GPa for the samples PE1 produced from the steel E according to the invention by ultrasonic means subject to the sound-propagation velocity. In contrast, the elasticity module of the conventional samples PV1 was 294 GPa.

Table 3 gives an overview of the thermal expansion of the steel E. This was measured by means of a Bähr dilatometer in temperature steps of 100° C. up to a maximum temperature of 600° C. It can be identified that the thermal coefficient of expansion α^(th) lies in this temperature range between 7 and 12 10-⁶/K. In addition to this, FIG. 4 shows, by way of example, the result of a dilatometer measurement on a sample PE1 produced from the steel according to the invention, which confirms this result.

TABLE 1 Steel Cr Mo Ni Co Cu Ti Al TiC NbC E 13.5 7.0 5.0 9.0 0.8 1.0 1.2 33 2.5 V 13.5 5.0 4.0 9.0 0.8 1.0 1.0 30 — Data in % wt., remainder iron and unavoidable impurities

TABLE 2 Sample Average hardness HRC PE1 65 PV1 62 PE2 54 PV2 53

TABLE 3 Temperature [° C.] α^(th) 100 8.4 200 8.7 300 9.0 400 9.2 500 9.4 600 9.7 

1.-18. (canceled)
 19. A steel having high wear resistance, a high degree of hardness, good corrosion resistance and/or low thermal conductivity, wherein the steel has a hardness of at least 56 HRC in a hardened state, wherein in a microstructure of the steel in total at least 30% wt. of hard phases are present which in addition to TiC particles include further carbide particles, oxide particles or nitride particles, wherein a content of TiC particles is at least 20% wt. and 2-4.5% wt. of NbC particles are present, and wherein the hard phases are embedded in a matrix which consists (in % wt.) of 9.0-15.0% Cr, 5.0-9.0% Mo, 3.0-7.0% Ni, 6.0-11.0% Co, 0.3-1.5% Cu, 0.1-2.0% Ti, 0.1-2.0% Al, and a remainder being iron and unavoidable impurities.
 20. The steel according to claim 19, herein its Cr content is 12.5-14.5% wt.
 21. The steel according to claim 19, wherein its Mo content is 6.5-7.5% wt.
 22. The steel according to claim 19, wherein its Ni content is 4.5-5.5% wt.
 23. The steel according to claim 19, wherein its Co content is 8-10% wt.
 24. The steel according to claim 19, wherein its Cu content is 0.5-1.0% wt.
 25. The steel according to claim 19, wherein its Ti content is 0.8-1.2% wt.
 26. The steel according to claim 19, wherein its Al content is 1.0-1.4% wt.
 27. The steel according to claim 19, wherein its TiC content is at most 45% wt.
 28. The steel according to claim 19, wherein the steel is produced by powder metallurgy. 